Simulations make earthquake hazards less shaky

We may never predict earthquakes, but computer models are putting emergency response plans on much more solid ground. In a talk not far from one of the country's seismic hot spots, seismologist Thomas Jordan of the Southern California Earthquake Center discussed "Understanding Earthquakes Through Large-Scale Simulations" on 20 February at the 2010 AAAS meeting in San Diego.

"We can learn lessons from these simulated events that often have to be learned the hard way through tragedy and experience," said Jordan, who directs the center.

Sophisticated simulations now play key roles in planning how to respond to earthquakes, Jordan said. For instance, more than 5 million people participated in the Great California ShakeOut in November 2008, the largest emergency response exercise in U.S. history. During the exercise, emergency response units reacted to the projected effects of a magnitude 7.8 earthquake along the southern San Andreas fault. The ShakeOut's events drew upon detailed models of seismic activity and the fault's behavior.

"This scenario demonstrated that existing disaster plans for an earthquake of this magnitude are inadequate and motivated us to re-examine our emergency response," Jordan said. "Large exercises like this are the best way to prepare for large earthquakes."

Researchers continually try to improve their simulations, but earthquake faults react to a suite of entangled forces and stresses, Jordan said.

"Earthquakes are system-level phenomena that emerge from complex long-term interactions within active fault systems," Jordan said. "They cascade as chaotic chain reactions. This very random behavior makes them difficult to predict."

Instead of trying to predict the exact time and duration of earthquakes, Jordan is working toward an accurate probability estimate to reduce the hazards they pose. By investigating fault interactions, the behavior of seismic waves, seismic history, and thousands of other known factors, Jordan and his colleagues can refine projections for the odds that earthquakes will strike in southern California within a few years or a few decades.

As seismic waves travel outward from the fault rupture, they dissipate—but not uniformly. For example, changes in rock types cause waves to bend and amplify in the Los Angeles area, which sits in the San Fernando Valley.

By tapping a three-dimensional model of the geology underlying southern California's terrain, scientists used supercomputers to generate models for what might happen during earthquakes of various sizes and locations.

"We use these models to generate simulations of earthquakes and understand how the waves bounce around and predict their behavior," Jordan said. "These are bowls of jelly that shake very hard during earthquakes. Basins are places where we have real problems."

The goal of these simulations is operational earthquake forecasting, Jordan said. Exact prediction probably will elude science, but researchers project that an earthquake of greater than magnitude 6.7 is much more likely in southern California than in the San Francisco Bay Area within the next 30 years.

"The northern section ruptured in 1906, which relieved that strain, whereas in southern California, we haven't had a large earthquake on the San Andreas since 1857. It's had more time for strain to accumulate," Jordan said. "The entire southern San Andreas is locked and loaded and ready to go."

Based on simulations of the San Andreas, disastrous seismic waves would reach Los Angeles about 85 seconds after the fault ruptured. People will have mere moments of warning. However, automated safety systems should prevent some deaths and injuries, Jordan believes.

Jordan made it clear that every second counts in a quake's aftermath: "For a society that runs on a millisecond clock, even small gains in probabilities and small gains in warnings can be used to improve our resilience to earthquake disasters."

#

Elizabeth Storey is a senior at the University of Tennessee, Knoxville, majoring in geology and journalism. Reach her at estorey@utk.edu.

Feb. 24, 2010

Amplify the signal